1. Field of Invention
This invention relates to screw feeds for machine tools and in particular to a D.C. motor driven screw feed for the table of a machine tool.
2. Prior Art
D.C. motor driven screw type table feeds for machine tools are well known. Along with hydraulic means, these D.C. motor driven feed apparatuses have superior characteristics as apparatuses for controlling the feed of machine tools.
As shown in FIG. 1, conventional feed driving motors cause a feed screw 3 to revolve via a coupling 6, but do not contain any thrust bearings which are able to bear a load. Instead, the load of the thrust which accompanies the linear motion of the slide 2 acts upon the feed screw 3 and is borne by the thrust bearings 5 installed in column 4 so that the load does not act upon the feed driving motor 1. Such conventional installation of thrust bearings 5 in column 4 have the following disadvantages: It increases the overall number of parts in the feed and it makes maintenance of the feed complicated and difficult.
Furthermore, conventional motor driven feeds have an additional problem in that the feed screw collects dirt and requires lubrication and therefore must be provided with a protecting cover. Conventional methods for protecting the feed screws of machine tools, etc. have consisted of protecting the feed screw indirectly by placing a cover over the entire feed slide mechanism or covering the circumference of the feed screw with a helical spring type cover described in Japanese Pat. No. 638,408. Such a helical spring type cover is commercially referred to as "elasticone".
In such configurations, however, the feed screw is not completely protected and no consideration whatever has been given to lubrication of the feed screw. Conventional protective covers in which consideration has also been given to lubrication of the feed screw as shown in FIGS. 2 and 3. In these protective covers 7, the central axis of the cover 7 coincides with the central axis of the screw 8 so that a so-called grease pocket 7A is formed by the space between the cover 7 and the feed screw 2. If grease, for example, is used as a lubricant for the feed screw 8, this grease is dispersed by a centrifugal force ans the feed screw 8 revolves. Accordingly, it is desirable that the volume of the grease pocket be small, as shown in FIG. 2, in order to keep the grease stored inside the protective cover 7 in constant contact with the feed screw 8 so that sufficient lubrication action is obtained. On the other hand, however, when the volume of the grease pocket is small, the heat capacity of the contained grease is small so that the grease is easily liquified. Since the absolute quantity of grease is small, it becomes necessary to frequently replenish the grease. Accordingly, a conflict arises since it is also desirable that the volume of the grease pocket be large, as shown in FIG. 3. Thus, conventional protective covers for feed screws have suffered from a drawback in that in these covers it is difficult to adequately solve both the problem of centrifugal force acting upon the lubricant and the problem of heat capacity.
In addition, conventional D.C. motor control apparatuses with motor driven screw feeds which lack positional feedback have a particular drawback. This drawback is its inability to correspond well to the rate changing device. Specifically, although the rate of feed of the table is changed on account of a reduction in cycle time, during feed movement these apparatuses have not been able to immediately cause the rate of revolution of the D.C. motor to conform to the rate ordered when an order to change the rate is sent to the D.C. motor. It is well known that the ability to respond is especially poor in cases where the inertia and weight of the table which includes the main shaft unit or work table are great. Accordingly, conventional rate changing orders for which conventional limit switches are employed have caused drawbacks such as table overdrive, etc.
This invention has been designed with the above described conventional problems in mind.